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<title>Struct template pass_through</title>
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<div class="refentry" title="Struct template pass_through">
<a name="boost.proto.pass_through"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2><span class="refentrytitle">Struct template pass_through</span></h2>
<p>boost::proto::pass_through &#8212; A <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a> that transforms the child expressions of an expression
          node according to the corresponding children of a Grammar.</p>
</div>
<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="emphasis"><em>// In header: &lt;<a class="link" href="../../proto/reference.html#header.boost.proto.transform.pass_through_hpp" title="Header &lt;boost/proto/transform/pass_through.hpp&gt;">boost/proto/transform/pass_through.hpp</a>&gt;

</em></span><span class="bold"><strong>template</strong></span>&lt;<span class="bold"><strong>typename</strong></span> Grammar&gt; 
<span class="bold"><strong>struct</strong></span> <a class="link" href="pass_through.html" title="Struct template pass_through">pass_through</a> : <span class="bold"><strong></strong></span> <span class="type"><a class="link" href="transform.html" title="Struct template transform">proto::transform</a>&lt; pass_through&lt;Grammar&gt; &gt;</span> {
  <span class="emphasis"><em>// member classes/structs/unions</em></span>
  <span class="bold"><strong>template</strong></span>&lt;<span class="bold"><strong>typename</strong></span> Expr, <span class="bold"><strong>typename</strong></span> State, <span class="bold"><strong>typename</strong></span> Data&gt; 
  <span class="bold"><strong>struct</strong></span> <a class="link" href="pass_through/impl.html" title="Struct template impl">impl</a> : <span class="bold"><strong></strong></span> <span class="type"><a class="link" href="transform_impl.html" title="Struct template transform_impl">proto::transform_impl</a>&lt;Expr, State, Data&gt;</span> {
    <span class="emphasis"><em>// types</em></span>
    <span class="bold"><strong>typedef</strong></span> <span class="bold"><strong>typename</strong></span> proto::result_of::child_c&lt;Grammar, N&gt;::type <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.GN">GN</a>;           <span class="emphasis"><em>// For each N in [0,Expr arity), for exposition only</em></span>
    <span class="bold"><strong>typedef</strong></span> <span class="bold"><strong>typename</strong></span> proto::result_of::child_c&lt;Expr, N&gt;::type    <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.EN">EN</a>;           <span class="emphasis"><em>// For each N in [0,Expr arity), for exposition only</em></span>
    <span class="bold"><strong>typedef</strong></span> <span class="bold"><strong>typename</strong></span> boost::result_of&lt;GN(EN,State,Data)&gt;::type   <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.RN">RN</a>;           <span class="emphasis"><em>// For each N in [0,Expr arity), for exposition only</em></span>
    <span class="bold"><strong>typedef</strong></span> <span class="bold"><strong>typename</strong></span> Expr::proto_tag                             <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.T">T</a>;            <span class="emphasis"><em>// For exposition only</em></span>
    <span class="bold"><strong>typedef</strong></span> <a class="link" href="expr.html" title="Struct template expr">proto::expr</a>&lt;T, <a class="link" href="listN.html" title="Struct template listN">proto::listN</a>&lt;R0,...RN&gt; &gt;              <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.result_type">result_type</a>;

    <span class="emphasis"><em>// <a class="link" href="pass_through/impl.html#id1271854-bb">public member functions</a></em></span>
    <span class="type">result_type</span> <a class="link" href="pass_through/impl.html#id1271859-bb"><span class="bold"><strong>operator</strong></span>()</a>(<span class="bold"><strong>typename</strong></span> impl::expr_param, 
                           <span class="bold"><strong>typename</strong></span> impl::state_param, 
                           <span class="bold"><strong>typename</strong></span> impl::data_param) <span class="bold"><strong>const</strong></span>;
  };
};</pre></div>
<div class="refsect1" title="Description">
<a name="id1676146"></a><h2>Description</h2>
<p>
            Given a Grammar such as <code class="computeroutput"><a class="link" href="plus.html" title="Struct template plus">proto::plus</a>&lt;T0, T1&gt;</code>,
            an expression type that matches the grammar such as
            <code class="computeroutput"><a class="link" href="plus.html" title="Struct template plus">proto::plus</a>&lt;E0, E1&gt;::type</code>, a state
            <code class="computeroutput">S</code> and a data <code class="computeroutput">D</code>, the result of applying
            the <code class="computeroutput">proto::pass_through&lt;<a class="link" href="plus.html" title="Struct template plus">proto::plus</a>&lt;T0, T1&gt; &gt;</code>
            transform is: </p>
<pre class="programlisting"><a class="link" href="plus.html" title="Struct template plus">proto::plus</a>&lt;
  boost::result_of&lt;T0(E0, S, D)&gt;::type,
  boost::result_of&lt;T1(E1, S, D)&gt;::type
&gt;::type</pre>
<p>
          </p>
<p>
            The above demonstrates how child transforms and child expressions are applied pairwise, and how the
            results are reassembled into a new expression node with the same tag type as the original.
          </p>
<p>
            The explicit use of <code class="computeroutput">proto::pass_through&lt;&gt;</code> is not usually
            needed, since the expression generator metafunctions such as
            <code class="computeroutput"><a class="link" href="plus.html" title="Struct template plus">proto::plus</a>&lt;&gt;</code> have
            <code class="computeroutput">proto::pass_through&lt;&gt;</code> as their default transform. So,
            for instance, these are equivalent:
            </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><code class="computeroutput">
                  <a class="link" href="when.html" title="Struct template when">proto::when</a>&lt; <a class="link" href="plus.html" title="Struct template plus">proto::plus</a>&lt;X, Y&gt;, proto::pass_through&lt; <a class="link" href="plus.html" title="Struct template plus">proto::plus</a>&lt;X, Y&gt; &gt; &gt;
                </code></li>
<li class="listitem"><code class="computeroutput">
                  <a class="link" href="when.html" title="Struct template when">proto::when</a>&lt; <a class="link" href="plus.html" title="Struct template plus">proto::plus</a>&lt;X, Y&gt;, <a class="link" href="plus.html" title="Struct template plus">proto::plus</a>&lt;X, Y&gt; &gt;
                </code></li>
<li class="listitem"><code class="computeroutput">
                  <a class="link" href="when.html" title="Struct template when">proto::when</a>&lt; <a class="link" href="plus.html" title="Struct template plus">proto::plus</a>&lt;X, Y&gt; &gt; // because of proto::when&lt;class X, class Y=X&gt;
                </code></li>
<li class="listitem"><code class="computeroutput">
                  <a class="link" href="plus.html" title="Struct template plus">proto::plus</a>&lt;X, Y&gt;  // because plus&lt;&gt; is both a grammar and a transform
                </code></li>
</ul></div>
<p>
          </p>
<p>
            For example, consider the following transform that promotes all
            <code class="computeroutput">float</code> terminals in an expression to
            <code class="computeroutput">double</code>.
            </p>
<pre class="programlisting">// This transform finds all float terminals in an expression and promotes
// them to doubles.
struct Promote :
  <a class="link" href="or_.html" title="Struct template or_">proto::or_</a>&lt;
    <a class="link" href="when.html" title="Struct template when">proto::when</a>&lt;<a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a>&lt;float&gt;, <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a>&lt;double&gt;::type(<a class="link" href="_value.html" title="Struct _value">proto::_value</a>) &gt;,
    // terminal&lt;&gt;'s default transform is a no-op:
    <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a>&lt;<a class="link" href="_.html" title="Struct _">proto::_</a>&gt;,
    // nary_expr&lt;&gt; has a pass_through&lt;&gt; transform:
    <a class="link" href="nary_expr.html" title="Struct template nary_expr">proto::nary_expr</a>&lt;<a class="link" href="_.html" title="Struct _">proto::_</a>, <a class="link" href="vararg.html" title="Struct template vararg">proto::vararg</a>&lt;Promote&gt; &gt;
  &gt;
{};</pre>
<p>
          </p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2008 Eric Niebler<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
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